The present invention relates to rolling bearings suitable for supporting rotating spindles used in devices for information systems, such as a hard disk drive, and to methods of producing these rolling bearings. More particularly, the invention relates to rolling bearings capable of efficiently preventing vibrations and acoustic noise generation as well as to methods of producing such rolling bearings.
Bearings used in a driving motor for a hard disk drive which serves as a magnetic storage medium in information systems and those used in a driving motor for a polygon mirror of a laser printer, for example, have a compact structure and are operated under relatively light loads. In these applications, it is required that vibrations and acoustic noise produced by the bearings themselves when a spindle supported by the bearings is rotated at a high speed be maintained to the lowest possible level. A conventional technique used for meeting this requirement is to achieve desired levels of surface hardness by quenching and tempering bearing steels and surface finish.
There is a growing tendency to smaller information systems in recent years. With the proliferation of transportable and portable information systems, they are used under ever diversified environmental conditions. Consequently, bearings built in these systems are often operated after they have been left in a non-operating condition for a long period of time in high-temperature environments of 50.degree. C. or above. When left at such high ambient temperatures, the bearings tend to produce significantly increased levels of mechanical and acoustic vibrations. The transportable and portable information systems could be accidentally dropped or caused to hit against a nearby object during transportation. This kind of mechanical shocks can cause the bearings to generate high-level acoustic noise in later use. The useful life of a bearing incorporated in the information systems and other precision machinery and equipment terminates when acoustic noise from the bearing exceeds the tolerable level of operating personnel, rather than when the rolling contact fatigue life of the bearing expires. It is therefore essential to take improvement measures to prevent degradation in acoustic performance of the bearing.
The most likely cause of acoustic performance degradation is a phenomenon which will be described below. If a bearing is left in a non-operating condition for such a long time as 100 hours at high temperatures ranging from 60.degree. C. to 90.degree. C., for example, stresses concentrated on contact surfaces between rolling elements (e.g., steel balls) and inner and outer rings will create as many concave indentations as the number of the rolling elements in the rolling contact surfaces of the bearing rings, even though these indentations are extremely shallow. It is supposed that acoustic vibrations occur when the bearing is operated in this condition, because the rolling elements roll over the successive indentations in a synchronized sequence. Similar concave indentations can also be created on the rolling contact surfaces of the inner and outer rings which are kept in contact with the rolling elements when equipment containing the bearing is dropped or hit against a surrounding object. This can also cause degradation in the acoustic performance of the bearing.
The indentations which can result in degradation in the acoustic performance of the bearing are created by the stresses concentrated at contact points between the individual bearing rings and rolling elements as stated above. The amount of stress concentrated at each contact point is usually half or less the stress level corresponding to the elastic limit of steel elements of the bearing and, therefore, no indentations are supposed to be formed from a theoretical point of view. An impact force that acts on a bearing when equipment containing it is caused to hit against an object is not so severe, as it is no more than one-third the static load carrying capacity of the bearing. However, extremely small-sized indentations formed by such a low level of stress can cause degradation in the acoustic performance of the bearing used in high-precision information systems, although such a low-level stress is likely to be considered negligible in most cases.
A technique used for achieving desired hardness and impact resistance in conventional rolling type bearings involves quenching of bearing steels and tempering at a relatively low temperature of 200.degree. C. or less, in which a certain amount of retained austenite remaining after the quenching process is left as it is. It is, however, recognized today that a steel product whose retained austenite content has been reduced to 0% in second stage tempering is less prone to the formation of indentations due to plastic deformation and is best suited as a bearing material for high-precision equipment, when compared to the conventional bearing steels containing a small amount of retained austenite ("Transactions ASME J. Basic Engineering" June (1960), p. 302; "Bearing" vol. 25 (1983), p. 23). Another known technique employed in a recently developed bearing for preventing its acoustic performance degradation is to reduce the retained austenite content down to 6% by volume or less (e.g., Japanese Unexamined Patent Publication No. 7-103241).
However, even with the technique disclosed in the aforementioned Japanese Unexamined Patent Publication No. 7-103241, concentrated stresses between rolling elements (e.g., steel balls) and rolling contact surfaces of bearing rings that occur when the bearing is kept in a high-temperature environment ranging from 60.degree. C. to 90.degree. C. will cause gradual decomposition of unstable retained austenite. This will result in permanent deformation, or indentations, left in the rolling contact surfaces at their points of contact with the rolling elements. It is quite likely that small-sized indentations are created by an impact force acting on contact points between the rolling elements and the rolling contact surfaces of the bearing rings, because of the elastic limit of the retained austenite being lower than that of martensite. It can be expected from the foregoing that a reduction in the amount of retained austenite is one of effective approaches to preventing the above-described undesirable phenomenon. Even with the aforementioned techniques, however, it is not possible to satisfactorily prevent acoustic performance degradation resulting from the formation of indentations caused by concentrated stresses.